Soft contact lenses made from thin high water content hydrogels are prone to corneal staining. Besides other factors, water flux through hydrogels and tonicity of the post lens tear film are believed to contribute to corneal staining. It would therefore be desirable to quantify in-vitro flux and diffusion coefficients of sodium chloride and water through hydrogels and relate this to the degree of corneal staining.
U.S. Pat. No. 5,776,999 to Nicolson et al. discloses a method for screening materials for use as extended-wear ophthalmic lenses.
A number of causes for disturbances to the epithelium secondary to hydrogel contact lens wear have been reported in the literature. These include mechanical irritation, chemical irritation and toxicity, epithelial edema and localized corneal drying. Epithelial staining may occur as a result of one or a combination of these factors.
The literature has also reported incidents of corneal epithelial erosions on wearing thin high water content hydrogel lenses. A comparison of corneal staining in these lenses (Permalens 71% water, Duragel 75% water) reveals the fact that the degree of corneal staining is related to the water content of these hydrogel lenses. Since the dehydration rate of the anterior surface of hydrogel lenses is proportional to their water content, it has been postulated that this evaporation draws water from the back of the lens to the front, leading to corneal damage. It has been suggested that there are patient factors, lens factors, and environmental factors that influence the occurrence of corneal staining with very thin high water content lenses. Thus it would be desirable to provide a method for selecting contact lens materials to reduce the incidence of corneal staining .
The invention also provides a method for selecting contact lens materials for contact lenses to reduce corneal staining. In a first embodiment, the invention provides a method for selecting contact lens materials to reduce corneal staining comprising the steps of:
(a) providing a sample of a contact lens material;
(b) determining the water diffusion coefficient of said hydrogel contact lens material;
(c) determining the sodium chloride diffusion coefficient of said contact lens material; and
(d) selecting said contact lens material to reduce corneal staining if the ratio of said water diffusion coefficient to said sodium chloride diffusion coefficient exceeds a predetermined value.
In a second embodiment, the method of the invention comprises:
(a) defining a calibration algorithm to relate the NMR spectra of at least three known concentrations H2O in solution with D2O, wherein the concentration of D2O is greater than the concentration of H2O;
(b) providing a first cell containing a first aqueous solution;
(c) providing a second cell containing a second solution comprising D2O;
(d) equilibrating a first sample of said contact lens material in D2O and positioning said equilibrated first sample of contact lens material at an interface between said first cell and said second cell to permit diffusion across said equilibrated contact lens material;
(e) measuring the NMR spectrum of said second solution;
(f) calculating the concentration of hydrogen ions in said second cell by applying the calibration algorithm of step (a) to said NMR spectrum of said second solution;
(g) calculating the water diffusion coefficient of said contact lens material sample based upon the hydrogen ion concentration calculated in step (f);
(h) providing a third cell containing a third aqueous solution containing a halide salt;
(i) providing a fourth cell containing a fourth aqueous solution such that the concentration of halide salt in said third aqueous solution is greater than the concentration of halide salt in said fourth aqueous solution;
(j) equilibrating a second sample of said contact lens material in water and positioning said second equilibrated sample of said contact lens material at an interface between said third cell and said fourth cell to permit diffusion across said second equilibrated sample of said contact lens material;
(k) withdrawing a sample of said fourth aqueous solution from said fourth cell;
(l) admixing said withdrawn sample of step (k) with a known quantity of an acid reagent;
(m) measuring the halide ion concentration of said admixed sample of step (l) by titrating said admixed sample by passing a substantially constant electric current between a silver-containing anode and a cathode in contact with said admixed sample;
(n) repeating the sampling and titration steps (k), (l) and (m) to obtain the halide ion concentration in said second fourth solution as a function of time;
(o) calculating the halide ion diffusion coefficient of said contact lens material using the halide ion concentrations obtained in step (n); and
(p) dividing the water diffusion coefficient of step (g) by the halide ion diffusion coefficient of step (o) to obtain a corneal staining ratio; and
(q) selecting said contact lens material to reduce corneal staining if the corneal staining ratio in step (p) exceeds a predetermined value.